An ESR spin-probe study of individual molecular reorientation vs. collective modes in supercooled phase V

We report on a line-shape study of angular-dependent ESR spectra from uniform samples of spiri-probe-doped supercooled phase V. Experiments were performed at -50°C, a temperature at which the liquid-crystalline director stops reorienting along the magnetic field of the ESR spectrometer, smectic-type layering sets in, and the paramagnetic dopants are immobile. Typical ESR spectra, generated by different functional forms of the spatial distribution of molecular orientations, are associated with distinct sites for locating additives, the main distinguishing qualifiers being the nature of the dynamic mode experienced by the probe at higher temperatures and the extent of solute-solvent interactive coupling. This study highlights the potential use of anisotropic rigid-limit ESR spectra to determine the spatial molecular distribution. It may have direct implications on the phenomenon of monotropic formation of liquid-crystalline mesophases.